Some recent image forming apparatuses such as printers, facsimiles, and copiers form images on continuous roll paper, thus increasing the overall printing speeds when compared with the case of forming images on cut paper. To form images on both sides of roll paper, the image forming apparatus may be implemented by a tandem type image forming apparatus in which a sheet reversing device is disposed between two image forming devices. Alternatively, the image forming apparatus may be implemented by a single engine duplex (SED) type image forming apparatus in which a single image forming device and a sheet reversing device are mainly provided, for example, as described in Japanese Patent Application Publication Nos. H08-2017 and 2001-253144.
More specifically, as described in Japanese Patent Application Publication No. H08-2017, a surface of a photoconductive drum that functions as the image forming device is divided into a first section for forming an image on one side of roll paper and a second section for forming an image on the other side of roll paper in the main-scanning direction. After the image is formed on the one side of roll paper using the first section of the photoconductive drum, the roll paper is turned over by the sheet reversing device. The roll paper is further transferred to a position that faces the second section of the photoconductive drum for image formation on the other side of roll paper.
In order to perform image formation on both sides of roll paper concurrently, the above-described SED type image forming apparatus is provided with a controller for controlling image formation on one side of roll paper and a controller for controlling image formation on the other side of roller paper. Since these two controllers need to be synchronized with each other, a structure of the controller or operation performed by the controller tends to become more complex. For example, in order to synchronize operations performed by the controllers, an additional interface is needed for each controller. On the other hand, it may be possible to cause a data input interface of the controller to perform synchronization operation. This, however, would lower the data transfer speeds, thus lowering the processing capability of the apparatus.